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.TH LOCKF 3C "Feb 16, 2015"
.SH NAME
lockf \- POSIX-style record locking on files
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>

\fBint\fR \fBlockf\fR(\fBint\fR \fIfildes\fR, \fBint\fR \fIfunction\fR, \fBoff_t\fR \fIsize\fR);
.fi

.SH DESCRIPTION
.LP
The \fBlockf()\fR function allows sections of a file to be locked; advisory or
mandatory write locks depending on the mode bits of the file (see
\fBchmod\fR(2)). Calls to \fBlockf()\fR from other threads that attempt to lock
the locked file section will either return an error value or be put to sleep
until the resource becomes unlocked. All the locks for a process are removed
when the process terminates. See \fBfcntl\fR(2) for more information about
record locking.
.sp
.LP
The \fIfildes\fR argument is an open file descriptor. The file descriptor must
have \fBO_WRONLY\fR or \fBO_RDWR\fR permission in order to establish locks with
this function call.
.sp
.LP
The \fIfunction\fR argument is a control value that specifies the action to be
taken. The permissible values for \fIfunction\fR are defined in
<\fBunistd.h\fR> as follows:
.sp
.in +2
.nf
#define   F_ULOCK   0   /* unlock previously locked section */
#define   F_LOCK    1   /* lock section for exclusive use */
#define   F_TLOCK   2   /* test & lock section for exclusive use */
#define   F_TEST    3   /* test section for other locks */
.fi
.in -2

.sp
.LP
All other values of \fIfunction\fR are reserved for future extensions and will
result in an error if not implemented.
.sp
.LP
\fBF_TEST\fR is used to detect if a lock by another process or open file handle
is present on the specified section. \fBF_LOCK\fR and \fBF_TLOCK\fR both lock
a section of a file if the section is available. \fBF_ULOCK\fR removes locks
from a section of the file.
.sp
.LP
The \fIsize\fR argument is the number of contiguous bytes to be locked or
unlocked. The resource to be locked or unlocked starts at the current offset in
the file and extends forward for a positive \fIsize\fR and backward for a negative
\fIsize\fR (the preceding bytes up to but not including the current offset). If
\fIsize\fR is zero, the section from the current offset through the largest
file offset is locked (that is, from the current offset through the present or
any future end-of-file). An area need not be allocated to the file in order to
be locked as such locks may exist past the end-of-file.
.sp
.LP
The sections locked with \fBF_LOCK\fR or \fBF_TLOCK\fR may, in whole or in
part, contain or be contained by a previously locked section for the same
process.  Locked sections will be unlocked starting at the point of the offset
through \fIsize\fR bytes or to the end of file if \fIsize\fR is (\fBoff_t\fR)
0. When this situation occurs, or if this situation occurs in adjacent
sections, the sections are combined into a single section. If the request
requires that a new element be added to the table of active locks and this
table is already full, an error is returned, and the new section is not locked.
.sp
.LP
\fBF_LOCK\fR and \fBF_TLOCK\fR requests differ only by the action taken if the
resource is not available. \fBF_LOCK\fR blocks the calling thread until the
resource is available. \fBF_TLOCK\fR causes the function to return \(mi1 and
set \fBerrno\fR to \fBEAGAIN\fR if the section is already locked by another
process.
.sp
.LP
File locks are released on first close by the locking process of any file
descriptor for the file.
.sp
.LP
\fBF_ULOCK\fR requests may, in whole or in part, release one or more locked
sections controlled by the process. When sections are not fully released, the
remaining sections are still locked by the process. Releasing the center
section of a locked section requires an additional element in the table of
active locks. If this table is full, an \fBerrno\fR is set to \fBEDEADLK\fR and
the requested section is not released.
.sp
.LP
An \fBF_ULOCK\fR request in which \fIsize\fR is non-zero and the offset of the
last byte of the requested section is the maximum value for an object of type
\fBoff_t\fR, when the process has an existing lock in which \fIsize\fR is 0 and
which includes the last byte of the requested section, will be treated as a
request to unlock from the start of the requested section with a \fIsize\fR equal to
0. Otherwise, an \fBF_ULOCK\fR request will attempt to unlock only the
requested section.
.sp
.LP
A potential for deadlock occurs if the threads of a process controlling a
locked resource is put to sleep by requesting another process's locked
resource. Thus calls to \fBlockf()\fR or \fBfcntl\fR(2) scan for a deadlock
prior to sleeping on a locked resource. An error return is made if sleeping on
the locked resource would cause a deadlock.
.sp
.LP
Sleeping on a resource is interrupted with any signal. The \fBalarm\fR(2)
function may be used to provide a timeout facility in applications that require
this facility.
.SH RETURN VALUES
.LP
Upon successful completion, \fB0\fR is returned.  Otherwise, \fB\(mi1\fR is
returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.LP
The \fBlockf()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 20n
The \fIfildes\fR argument is not a valid open file descriptor; or
\fIfunction\fR is \fBF_LOCK\fR or \fBF_TLOCK\fR and \fIfildes\fR is not a valid
file descriptor open for writing.
.RE

.sp
.ne 2
.na
\fB\fBEACCES\fR or \fBEAGAIN\fR\fR
.ad
.RS 20n
The \fIfunction\fR argument is \fBF_TLOCK\fR or \fBF_TEST\fR and the section is
already locked by another process.
.RE

.sp
.ne 2
.na
\fB\fBEDEADLK\fR\fR
.ad
.RS 20n
The \fIfunction\fR argument is \fBF_LOCK\fR and a deadlock is detected.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 20n
A signal was caught during execution of the function.
.RE

.sp
.ne 2
.na
\fB\fBECOMM\fR\fR
.ad
.RS 20n
The \fIfildes\fR argument is on a remote machine and the link to that machine
is no longer active.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 20n
The \fIfunction\fR argument is not one of \fBF_LOCK\fR, \fBF_TLOCK\fR,
\fBF_TEST\fR, or \fBF_ULOCK\fR; or \fIsize\fR plus the current file offset is
less than 0.
.RE

.sp
.ne 2
.na
\fB\fBEOVERFLOW\fR\fR
.ad
.RS 20n
The offset of the first, or if \fIsize\fR is not 0 then the last, byte in the
requested section cannot be represented correctly in an object of type
\fBoff_t\fR.
.RE

.sp
.LP
The \fBlockf()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 24n
The \fIfunction\fR argument is \fBF_LOCK\fR or \fBF_TLOCK\fR and the file is
mapped with \fBmmap\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBEDEADLK\fR or \fBENOLCK\fR\fR
.ad
.RS 24n
The \fIfunction\fR argument is \fBF_LOCK\fR, \fBF_TLOCK\fR, or \fBF_ULOCK\fR
and the request would cause the number of locks to exceed a system-imposed
limit.
.RE

.sp
.ne 2
.na
\fB\fBEOPNOTSUPP\fR or \fBEINVAL\fR\fR
.ad
.RS 24n
The locking of files of the type indicated by the \fIfildes\fR argument is not
supported.
.RE

.SH USAGE
.LP
Record-locking should not be used in combination with the \fBfopen\fR(3C),
\fBfread\fR(3C), \fBfwrite\fR(3C) and other \fBstdio\fR functions.  Instead,
the more primitive, non-buffered functions (such as \fBopen\fR(2)) should be
used.  Unexpected results may occur in processes that do buffering in the user
address space.  The process may later read/write data which is/was locked.  The
\fBstdio\fR functions are the most common source of unexpected buffering.
.sp
.LP
The \fBalarm\fR(2) function may be used to provide a timeout facility in
applications requiring it.
.sp
.LP
The \fBlockf()\fR function has a transitional interface for 64-bit file
offsets.  See \fBlf64\fR(7).
.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.LP
.BR Intro (2),
.BR alarm (2),
.BR chmod (2),
.BR close (2),
.BR creat (2),
.BR fcntl (2),
.BR mmap (2),
.BR open (2),
.BR read (2),
.BR write (2),
.BR attributes (7),
.BR lf64 (7),
.BR standards (7)
